Filtration is used to reduce certain particulates and/or vapour phase constituents of tobacco smoke inhaled during smoking. It is important that this is achieved without removing significant levels of other components, such as organoleptic components, thereby degrading the quality or taste of the product.
Smoking article filters are often composed of cellulose acetate fibres, which mechanically filter aerosol particles. It is also known to incorporate porous carbon materials into the filters (dispersed amongst the cellulose acetate fibres, or in a cavity in the filter) to adsorb certain smoke constituents, typically by physisorption. Such porous carbon materials can be made from the carbonized form of many different organic materials, most commonly plant-based materials such as coconut shell. However, synthetic polymers have also been carbonized to produce porous carbons. In addition, fine carbon particles have been agglomerated with binders to produce porous carbons, in the manner described in U.S. Pat. No. 3,351,071.
The precise method used to manufacture porous carbon material has a strong influence on its properties. It is therefore possible to produce carbon particles having a wide range of shapes, sizes, size distributions, pore sizes, pore volumes, pore size distributions and surface areas, each of which influences their effectiveness as adsorbents. The attrition rate is also an important variable; low attrition rates are desirable to avoid the generation of dust during high speed filter manufacturing.
Generally, porous carbons having a high surface area and large total pore volume are desired in order to maximise adsorption. However, this must be balanced with a low attrition rate. The surface area and total pore volume of conventional materials such as coconut carbons are limited by their relative brittleness. In addition, the ability to incorporate a large proportion of meso- and macropores is hindered by the strength of the material. As explained in Adsorption (2008) 14: 335-341, conventional coconut carbon is essentially microporous, and increasing the carbon activation time results in an increase in the number of micropores and surface area but produces no real change in pore size or distribution. Thus, it is generally not possible to produce coconut carbon containing a significant number of meso- or macropores.
Another factor to take into consideration is the fact that the residence time for smoke in a typical 27 mm-long cigarette filter, during standard measurements of tar content, is of the order of milliseconds. Thus, porous carbon materials for smoke filtration must be optimised to be very efficient adsorbers on such a short timescale.
In view of the foregoing, there is still room for improvement in the art with regard to the use of porous carbon materials for smoke filtration.